Polygon mirror motor and polygon mirror supporting structure

ABSTRACT

A polygon mirror assembly includes a polygon mirror and a supporting stand for supporting the polygon mirror. The supporting stand and the polygon mirror are combined in a pressure joining manner. The polygon mirror supporting structure can be provided so that the fastening of the polygon mirror is easily carried out, no unbalance of the center of rotation occurs and any eccentricity can be made extremely small.

CROSS REFERENCE TO EARLIER APPLICATION

This application is a continuation of application Ser. No. 08/675,068,filed Jul. 3, 1996, now U.S. Pat. No. 5,883,455, issued Mar. 16, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to a polygon mirror motor and a polygonmirror supporting structure; and, more particularly, to a polygon mirrormotor suitable for use, for example, in a laser beam printer or a lightwrite-in apparatus of an image forming apparatus, and to a polygonmirror installation supporting structure of a polygon mirror apparatus.

A polygon mirror motor is generally found in a laser beam printer and alaser beam scanning apparatus of an image forming apparatus. Thispolygon mirror motor carries a polygon mirror fixed to a rotative shaftof a brushless motor, and such a polygon mirror provides a mirrorsurface on the side of a member which is formed in a regular polygonshape, such as a six angle regular shape or an eight angle regularshape.

A laser light beam irradiates the rotating polygon mirror portion, whichreflects the light onto a photosensitive body, causing it be scannedthereby, so that a light write-in operation is carried out. In such apolygon mirror motor structure, to perform an accurate scanningoperation, it is necessary to maintain a right angle between therotative shaft and the polygon mirror, which often becomes a problem. Toavoid this problem, the method used for fixing the mirror to therotative shaft, or to a flange portion which is fixed to the rotativeshaft, becomes important.

As conventional mirror securing methods, there are various techniqueswhich are described in Japanese patent laid-open publication No. Sho62-80,616, Japanese patent laid-open publication No. Sho 61-175,612,Japanese utility model laid-open publication No. Hei 3-45,073, Japaneseutility model laid-open publication No. Hei 2-134,513, Japanese utilitymodel laid-open publication No. Hei 2-64,915, Japanese utility modellaid-open publication No. Sho 6432,513 and Japanese utility modellaid-open publication No. Sho 63-198,022.

As proposed in the above stated various prior techniques, the mirror isinstalled against a face of a rotating member supporting body, which isfixed to the rotative shaft using a screw member securing method, aspring member pressing method, an adhering method, an elastic bodyinterposition method or a wedge securing method, etc.

However, in the screw member securing method, according to theconventional technique, during the screw member fastening operation, avariation in the fastening torque due to a variation of the componentaccuracy resulting from manufacturing tolerances at each screw memberportion is unavoidable. Further, since screw members and metal washersare necessary, the number of components increases, and since a tap holeprocessing for providing a hole in the supporting member for the mirroris necessary, the manufacturing process is complicated and the componentaccuracy is inferior.

In the spring member pressing method, according to the conventionaltechnique, the defects of the screw member fixing method are avoided,however the pressure force applied to the mirror changes according to avariation of the spring constant of the spring member. Namely, when thepressure force applied to the mirror is weak, the mirror moves, and whenthe pressure force applied to the mirror is strong, the mirror willdeform.

In the adhering method, according to the conventional technique, anunbalance is generated due to a non-uniformity in the amount of appliedcoating and a dispersion of non-hardened liquid. Further, the mirrordeforms in response to heat generated during the high speed rotation ofthe polygon mirror motor, and this causes an unbalance.

In the elastic member interposing method, according to the conventionaltechnique, the elastic member tends to creep due to thermal stress, andthe pressure force provided via the elastic member becomes weak, thusthe mirror moves.

In the above stated mirror securing methods, including the wedgesecuring method, components in addition to the mirror and the mirrorinstallation metal seats are necessary, so that the assembly includingthe polygon mirror motor has a high cost.

The problems in the various conventional techniques for securing thepolygon mirror to the rotating member supporting body make it difficultto accurately maintain the mirror face in a regular polygon shape. Tomaintain the accuracy of the mirror face and to prevent a deformationthereof, between the rotating member supporting body and the mirror,other components are interposed, so that a high processing cost and highcomponent costs are necessary.

Further, since many components, in addition to the rotating membersupporting body and the mirror, are used and fixed to the motor shaft,an unbalance is generated due to a mass difference between differentportions of the assembly, and this causes a vibration during high speedrotation.

Also, with a construction having the above stated components, a changein the components with time due to high speed rotation is unavoidable,thereby the life of the polygon mirror motor is remarkably shortened.

The above stated conventional techniques for fixing the rotating membersupporting body to the regular polygon shape mirror, which is opposed toand in contact with the rotating member supporting body, have seriousinconveniences. The problems to be solved to eliminate theinconveniences in the conventional techniques reside in the need toestablish a predetermined accuracy of the mirror face, the increase in anumber of the components, the increase in unbalance due to thevariations resulting from manufacturing tolerances of the components, aregeneration in unbalance due to a change in the components with time,the increase in the regeneration in unbalance due to changes which occurover time, the shortening in the life of the assembly due to vibrationand a lowering in performance by unstable reflection of the laser beamlight due to distortion of the mirror face.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a polygon mirror motorand a polygon mirror supporting structure wherein no component isinterposed between the polygon mirror and a rotor supporting member forsupporting the polygon mirror, the polygon mirror and the rotorsupporting member being combined using a pressure joining method.

According to the present invention, a polygon mirror assembly comprisesa polygon mirror and a supporting stand for supporting the polygonmirror, the supporting stand and the polygon mirror being combined usinga pressure joining method.

In particular, a polygon mirror assembly comprises a polygon mirrorhaving a mirror arranged around the outer periphery thereof and asetting hole at a central portion, and a supporting stand having a bossportion for fitting into the setting hole of the mirror, the bossportion of the supporting stand and the setting hole of the polygonmirror being joined using pressure.

In accordance with the present invention, since the boss portion of thesupporting stand and the setting hole of the polygon mirror are joinedtogether using pressure, it is unnecessary to use screw members forfastening. As a result, it is unnecessary to carry out an assemblyprocess for fastening screw members or an adjustment of the fastening,which is difficult to achieve. Further, since holes for screw membersand the screw members are unnecessary, there is no variation in theweight with respect to the center of the combined rotating member andpolygon mirror, and so any eccentricity of the polygon mirror assemblyis extremely small.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a longitudinal cross-sectional view showing one embodiment ofa polygon mirror motor according to the present invention;

FIG. 2 is a plan view as seen from an upper portion of the polygonmirror of FIG. 1;

FIG. 3 is a longitudinal cross-sectional view of the polygon mirror anda portion of the rotating body;

FIG. 4 is a longitudinal cross-sectional view of a conventionalarrangement similar to the arrangement shown in FIG. 3;

FIG. 5 is a partial enlarged cross-sectional view showing anotherexample of a pressure combination state produced by a pressurecombination tool according to the present invention;

FIG. 6 is a longitudinal cross-sectional view of the polygon mirror anda rotating body before the pressure combination is carried out;

FIG. 7 is a partial enlarged cross-sectional view showing a pressurecombining portion wherein the pressure combination is carried out; and

FIG. 8 is a graph showing the relationship between mirror face accuracyand relaxation torque.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Hereinafter, one embodiment of a polygon mirror motor and a polygonmirror supporting structure according to the present invention will beexplained with reference to the drawings.

First of all, a polygon mirror motor will be explained with reference toFIG. 1, FIG. 2 and FIG. 3. The polygon mirror motor comprises a statorand a rotor. The stator of the polygon mirror motor comprises an outercasing 1, a stator core 2 and a bearing base 3. The stator core 2 isfixed to the inner periphery of the outer casing 1. A stator coil 4 isprovided on the stator core 2. The bearing base 3 is fixed to the openlower end of the outer casing 1 by means of screw members 5. An uprightbearing support member 6 of cylindrical shape is formed on a centralportion of the bearing base 3.

An upper side bearing member 7 is provided on an upper side of thebearing support member 6 and a lower side bearing member 8 is providedon a lower end of the bearing support member 6. A magnetic fluidlubricating oil is received in an axial bore in the bearing supportmember 6 through which a rotative shaft 10 passes so as to lubricate asliding face of the upper side bearing member 7 and a sliding face ofthe lower side bearing member 8. A fluid seal 9 is provided on the upperside of the bearing member 7 and prevents an outflow of the lubricatingoil.

The rotative shaft 10 of the rotor of the polygon mirror motor isrotatively supported by the upper side bearing member 7 and the lowerside bearing member 8. A rotor supporting body 11 is fixedly supportedon the rotative shaft 10. The whole rotor supporting body 11 is made ofan aluminum material. A cylindrical portion 12 of the rotor supportingbody 11 is formed to surround the outer periphery of the bearing supportmember 6.

A permanent magnet 13 operating as an armature is fixed to the outerperiphery of the cylindrical portion 12 of the rotor supporting body 11.Another permanent magnet 14 for detecting the position of the rotor ismounted at a lower portion of the permanent magnet 13 and is fixed tothe outer periphery of the cylindrical portion 12 of the rotorsupporting body 11. The permanent magnet 13, serving as the armature ofthe motor, is fixed to the outer periphery of the cylindrical portion 12of the rotor supporting body 11 by any means by which a centering withrespect to the rotational center of the rotor can be easily established.

A circuit board 15 is separately supported from an inner face of thebearing base 3, and a Hall element 16 is proved on the circuit board 15.This Hall element 16 is closely spaced to the outer periphery of thecylindrical portion 12 diametrically opposite to the location of theposition detecting permanent magnet 14. The polygon mirror motor is abrushless motor, and the location of the position detecting permanentmagnet 14 is accurately detected by the Hall element 16 so that a highlyaccurate speed adjustment of the motor can be performed.

A supporting stand 17 and a boss portion 18 are formed on the upperportion of the rotor supporting body 11. The rotative shaft 10 is fixedto the center of the supporting stand 17 and the boss portion 18, sothat it is easy to carry out the centering of the shaft 10 with respectto the rotational center of the rotor. A polygon mirror 19 is installedon the supporting stand 17 and is properly positioned by the bossportion 18 of the rotor supporting body 11. This polygon mirror 19 isfixed to the supporting stand 17 and the boss portion 18 of the rotorsupporting body 11 using a pressure joining method, according to thepresent invention.

The polygon mirror 19 is enclosed by a mirror cover 21, which is fixedto the upper portion of the outer casing 1 by means of screw members 20.A glass window 22 is fitted into and set at a position opposing theouter periphery of the polygon mirror 19. A laser beam 23, which isirradiated from a laser diode, passes through the glass window 22 and isincident on the rotating polygon mirror 19. The laser beam light 23 isreflected and passes again through the glass window 22 and is imaged ona body so as to be scanned on the body.

The polygon mirror 19 usually has more than two refection faces (mirrorfaces). The polygon mirror 19 of this embodiment according to thepresent invention has six (6) reflection or mirror faces, and is formedwith a regular hexahedron shape. The polygon mirror 19 has an outerdiameter D1 of 36 mm and has a central hole with an inner diameter D2 of20 mm, as shown in FIG. 2. The above inner diameter D2 of the polygonmirror 19 substantially coincides with the outer diameter of the bossportion 18 of the rotor supporting body 11. The polygon mirror 19 ismade of the same kind of an aluminum material as the rotor supportingbody 11. However, the purity of the aluminum material of the polygonmirror 19 is higher than that of the rotor supporting body 11. Thepurity of the aluminum material of the polygon mirror 19 is 99.99%degree.

The regular hexahedron shaped polygon mirror 19 is secured to the rotorsupporting body 11 using a pressure joining method. There are twomethods of effecting such a pressure joining according this invention.One of the two pressure joining methods is effected by using only pressworking under applied pressure. Another one of the methods is avibrating pressure joining method carried out by press working underapplied pressure with the addition of a vibration having a highvibrating frequency using a supersonic wave, for example.

First of all, the pressure joining method using only press working underapplied pressure will be explained. Using a pressure combination toolhaving a tip shaped as a pointed wedge with a sharp edge, pressure isapplied downward in the vicinity of the outer periphery of the bossportion 18 of the rotor supporting body 11 at spaced circumferentialpositions around the rotative shaft 10. As seen in FIG. 2, grooves 24are formed in the boss portion 18 of the rotor supporting body 11 by theedge of the pressure combination tool. With the formation of the grooves24, material is deformed outward from an outer peripheral portion toform a pressing projection 25 which overlays and presses downwardagainst an upper side edge portion of the setting hole 26 of the polygonmirror 19 to secure it into position.

By forming the pressing projection 25, the polygon mirror 19 is fastenedto the rotor supporting body 11. The pressure joining method isperformed at six circumferentially spaced positions, as shown in FIG. 2,and this causes pressure to be applied equally against the adjacentmirror face around the circumference of the shaft 10. The six pressureapplying projections are provided with an equal interval and these sixpressure projections are formed at one time. The above stated pressurejoining method is carried out using a pressure combination tool havingsix edges equally spaced wedge-like.

In one embodiment according to the present invention, as shown in FIG.3, the pressing projections 25 produced by the pressure joining methodoverlays an upper side edge portion of the setting hole of the polygonmirror 19. Therefore, the supporting stand 17 for supporting the polygonmirror 19 from a lower face thereof is not required to have a planesurface extending for a substantial distance in the radial direction.

In contrast, in the screw member fastening method of the prior art, asshown in FIG. 4, it is necessary to provide a large plane surfaceextending in the radial direction to form a supporting stand 117 for themirror 119. In this regard, the upper face of the rotor supporting body111 which forms the supporting stand 117 extends all the way to theouter periphery thereof from a boss portion 118 of the rotor supportingbody 111. In the above stated screw member fastening method of the priorart, a polygon mirror 119 is fastened to the rotor supporting body 111using screw members 127 extending through a metal seat 126, whichengages with an upper face of the polygon mirror 119.

Thereby, it is necessary to make the supporting stand 117 extend all theway to the outer periphery thereof from the boss portion 118 of therotor supporting body 111.

In FIG. 3, l₁ indicates a length in the radial direction of thesupporting stand 17 of one embodiment according to the presentinvention, and 13 indicates a length in the radial direction of thepolygon mirror 19. On the other hand, in FIG. 4, l₂ indicates a lengthin the radial direction of the supporting stand 117 according to theprior art, and l₄ indicates a length from the outer periphery of thepolygon mirror 119 to the screw member 127. The relationship between thelength l₁, the length l₂, the length l₃, and the length l₄ is regulatedso that the length l₁ <the length l₂ (l₁ <l₂), and so that the lengthl₃ > the length l₄ (l₃ >l₄). In one embodiment according to the presentinvention, the height of the boss portion 18 of the rotor supportingbody 11 is greater than the thickness of the polygon mirror 19.

The pressure joining method according to the present invention, asstated above, has the following advantages in comparison with the screwmember fastening method according to the prior art.

(1) It is unnecessary to provide a hole for the screw member, a metalseat, and a screw member, and it is unnecessary to perform assembly ofthe screw member components.

(2) It is unnecessary to perform a minute adjustment of the pressure ofthe screw member. Herein, when the fastening pressure for the screwmember is excessively strong, the mirror face positional accuracybecomes low due to a bending back deformation of the polygon mirrorresulting from fastening stress. On the other hand, when the fasteningpressure for the screw member is weak, the fastening of the polygonmirror 19 to the rotor supporting body 11 is insufficient, which causesvibration of the mirror as it rotates. However, using the pressurejoining method according to the present invention, the above statedminute adjustment is unnecessary, whereby the assembly of the polygonmirror 19 and the rotor supporting body 11 can be easily carried out.

(3) It is unnecessary to provide a hole for a screw member to fasten thepolygon mirror 19 to the rotor supporting body 11, and further it isunnecessary to use a screw member for fastening. Therefore, anyunbalance of the rotor, including the polygon mirror 19 and the rotorsupporting body 11, will be extremely small. Namely, the holes for thescrew members are to be arranged with an equal interval and the holeprocessing is performed so as to form the holes at positions having anequal distance from the rotational center of the rotor. In this regard,any deviation of the position for making the hole for a screw member maygenerate an unbalance. Further, in the screw member fastening method ofthe prior art, a deviation in weight of the respective screw membersalso inevitably causes, to some degree, an unbalance. However, in thepressure joining method employed according to the present invention,since the processing for making screw holes is unnecessary and the screwmembers and a metal washer are unnecessary, no unbalance of the typeinherent in the prior art is generated. Even when an unbalance results,such unbalance is extremely small. Since an unbalance is not generatedin the rotor, the life of the bearing members is increased, and since arotational unevenness is not generated, the dot interval of a scanningline of the polygon mirror 19 is made more uniform.

(4) As shown in FIG. 2, the pressure joining method is performed at sixpositions and these six positions are disposed on radial lines whichextend toward respective corners of the adjacent mirror faces.Therefore, an affect of warp of the mirror face can be mitigated.Namely, according to one embodiment of the present invention, since thepositions at which pressure is applied are disposed on radial linesextending toward a respective corner portion of the mirror face, thestress produced by the applied pressure is directed away from the centerof the mirror face. Accordingly, since the positions toward whichpressure is applied are separated remotely, the affect of warp on themirror face due to the applied pressure can be mitigated.

(5) Since plural pressure points having an equal interval are formed atone time by the pressure combination tool having plural edges, thepositioning of the polygon mirror 19 coincident with the rotationalcenter of the rotor supporting body 11 can be easily attained. The bossportion 18 of the rotor supporting body 11 is fitted into the settinghole of the polygon mirror 19. However, the diameter of the setting holeof the polygon mirror 19 is larger, to some degree, than the diameter ofthe boss portion 18 of the rotor supporting body 11, i.e. when it isobserved in detail, some gap exists. In the screw member fasteningmethod according to the prior art, the polygon mirror 119 is fastened tothe rotor supporting body 111 without regard to the eccentricityproduced by the above stated gap, since the positioning is determinedexclusively by the location of the screw holes. However, in oneembodiment according to the present invention, using a pressurecombination tool having plural tip pointed wedge shape edges arrangedwith an equal interval, the vicinity of the outer periphery of the bossportion 18 of the rotor supporting body 11 is subjected to an equalpressure around the periphery of the boss portion 18, so that aneccentricity does not result due to the above stated gap. Namely, whenthe pressure combination tool presses downward around the periphery ofthe rotative shaft 10, the edges of the pressure combination tool formthe grooves 24 on the boss portion 18 of the rotor supporting body 11.with the formation of the grooves 24, the pressing projection 25 isformed to extend out by deformation toward the outer peripheraldirection and overlay the edge of the mirror. This pressing projection25 is cut into the upper surface of the polygon mirror 19 and isdirected towards a side corner portion thereof so as to overlay the edgeof the setting hole of the polygon mirror 19. The pressure joiningmethod according to the present invention using cut-into working iscarried out by the formation of plural pressing projections 25, whichare simultaneously formed so as to press out over the edge of thesetting hole and also are arranged with an equal interval. Therefore,before the pressure joining operation, even when the boss portion 18 ofthe rotor supporting body 11 engages in the setting hole of the polygonmirror 19 with an eccentricity within a gap range, an eccentricityadjustment for amending the eccentricity can be carried out. Namely, inthe process wherein the pressing operation is carried out to form andpress out the pressing projection 25, the side of the polygon mirror 19is pressed out in the formation of the pressing projection 25 to enlargea part having a smaller gap and moves to a small extent. Therefore, apressure joining operation which also establishes permanent alignment ofthe polygon mirror 19 with the rotational center of the rotor supportingbody 11 can be carried out.

(6) Since both the polygon mirror 19 and the rotor supporting body 11are made of an aluminum material, which is soft, the use of thismaterial is well-suited to the pressure joining operation.

(7) Since the height of the boss portion 18 of the rotor supporting body11 is larger than the thickness of the polygon mirror 19, thefitting-into or setting portion which is cut into the upper side surfacethereof around the setting hole presses the polygon mirror 19 intodirect contact with the supporting stand 17 of the rotor supporting body11 from an upper oblique direction. Accordingly, the pressing projection25 performs a rotation prevention function in which the polygon mirror19 is prevented from rotation by the boss portion 18 of the rotorsupporting body 11 and performs a slip-out prevention function in whichthe polygon mirror 19 is prevented from slipping out from the bossportion 18 of the rotor supporting body 11. When the height of the bossportion 18 of the rotor supporting body 11 is smaller than the thicknessof the polygon mirror 19, it is possible to prevent the rotor supportingbody 11 from slipping out from the boss portion 18 of the rotorsupporting body 11, but by using a technique which creates anotherproblem.

For example, for preventing such slipping out, the polygon mirror 19 maybe formed with a step shape construction, allowing the upper side faceof the setting hole of the polygon mirror 19 to be lower than that ofthe boss portion 18 of the rotor supporting body 11. However, the stepshape of the above stated polygon mirror 19 is complicated andrepresents a weak structure.

(9) The polygon mirror 19 has a mirror face at the outer peripherythereof and a setting hole for engaging the boss portion 18 of the rotorsupporting body 11 at the central portion thereof. Further, the pressingprojections 25 are formed on the outer periphery of the boss portion 18of the rotor supporting body 11 according to the pressure joining methodso as to engage at the upper side edge of the setting hole of thepolygon mirror 19. Therefore, by using the pressure joining method,warping hardly appears on the mirror face of the polygon mirror 19. Inother words, as shown in FIG. 3, which shows one embodiment of thepolygon mirror and the rotary body according to the present invention,and FIG. 4, which shows the polygon mirror and the rotary body accordingto the prior art, the relationship between the length l₁, the length l₂,the length l₃, and the length l₄ is regulated so that the length l₁ <the length l₂ (l₁ <l₂) and to have the length l₃ > the length l₄ (l₃>l₄). As a result, according to one embodiment of the polygon mirror 19and the rotary body of the present invention, the distance from theportion where the pressing projections 25 engage the polygon mirror 19to the mirror face of the polygon mirror 19 is comparatively long, sothat the affect of warp due to the pressing operation on the polygonmirror 19 hardly appears at the mirror face of the polygon mirror 19.

(10) By pressing the upper face of the boss portion 18 of the rotor inthe vicinity of the setting hole of the polygon mirror 19, the pressingprojections 25 are cut away at the outer periphery of the upper end ofthe boss portion 18 of the rotor supporting body 11 so as to overlay andpress upon the mirror 19. Therefore, it is possible to effect a securingof the mirror 19 in a pressure joining manner.

However, when giving consideration to the affect of warp on the mirrorface of the polygon mirror 19, one embodiment according to the presentinvention as described above is superior, because the affect of the warphardly appears.

Next, a vibrating pressure joining method will be explained. This methodwill be explained mainly referring to FIG. 5, FIG. 6, FIG. 7 and FIG. 8.Except for the addition of an excitation source, the structure of thetool employed is basically the same as that of the pressure joiningmethod which uses only press working under applied pressure.

FIG. 5 shows a construction in which, in a vibrating pressure joiningapparatus, the polygon mirror 19 is fitted into and secured to the rotorsupporting body 11. A pressure combination tool 28 has six edges 29which are projected downward toward the face of the body 11. Theseplural edges 29 are arranged in a circular shape with an equal interval.The pressure combination tool 28 is made of a hard steel and the edges29 are formed by a hardening processing so that the edges 29 are madeharder than the other parts of the tool 28. The tip pointed edge 29 isformed with an oblique shape. An oblique face 30 is formed on a side ofan outer periphery of the pressure combination tool 28. A diameter whichis traced by the tip end of plural edges 29 is formed to be smaller thanthe diameter of the outer periphery of the boss portion 18 of the rotorsupporting body 11.

A pressurization source (not shown in the figure) and an excitationsource (not shown in the figure) are provided on the pressurecombination tool 28. The pressurization source applies a pressing forcetoward a lower direction and the excitation source adds a vibrationthereto. Except for the excitation source, this structure is the same asthe pressure joining arrangement used in the previously describedpressure joining method. The excitation source vibrates with a highvibration frequency, and in this embodiment a supersonic vibrating bodyis employed. The frequency is 15 KHz. The vibration direction of thisexcitation source is up and down. In other words, the pressurizationdirection of the pressurization source and the vibration direction ofthe excitation source are the same direction.

A receiving tool stand 31, as shown in FIG. 5, supports the rotorsupporting body 11 by engaging a lower face thereof. The pressurejoining method is carried out by pressing the pressure tool 28 from theupper side of the rotor supporting body 11, which is supported by thereceiving tool stand 31.

FIG. 6 shows a state in which the polygon mirror 19 is mounted on therotor supporting body 11. However, in the construction shown in FIG. 6,the application of pressure has not yet been carried out. The settinghole of the polygon mirror 19 is formed with a chamfering processing inwhich both the upper side edge portion and a lower side edge portion arechamfered. The chamfering width of the lower side edge chamfering C₂ is1 mm and the chamfering width of the upper side edge chamfering C₁ is0.3 mm.

FIG. 7 shows an enlarged portion relating to the pressure joining areaof FIG. 5. First of all, the sizes of each portion will be explained.The thickness T of the polygon mirror 19 is 4 mm. The length l₅ of thecontacting portion between the surface of the polygon mirror 19 and thesurface of the supporting stand 17 of the rotor supporting body 11 is 4mm. However, since the lower side chamfering C2 exists, the real lengthof surface contact with the supporting stand 17 is less than 4 mm.

The gap G existing between the outer periphery of the boss portion 18 ofthe rotor supporting body 11 and the inner periphery of the setting holeof the polygon mirror 19 is 0.01-0.02 mm. The depth of the groove MH is0.35 0.40 mm. The length M1 from a bottom portion of the groove 24 tothe outer periphery of the boss portion 18 of the rotor supporting body11 is 0.5 mm.

The groove 24 is an edge mark which is formed by the edges 29 of thepressure combination tool 28 and is formed with an inclined V shape. Theangle of the opening of the groove 24 is 35° and agrees with the tippointed angle of the edge 29. The upper side corner of the setting holeof the polygon mirror 29, over which the pressing projections 25 of theboss portion 18 of the rotor supporting body 11 is projected, forms aslope having an angle of 45°. This slope portion is part of the upperside chamfering C₁. A small range MS exists at a connection part atwhich the pressing projection 25 and the setting hole of the polygonmirror 19 are fitted together and secured to each other. In this smallrange, a metal combination occurs, which will be described in moredetail later.

Herein, the vibrating motion employed in the vibrating pressure joiningmethod will be explained. As shown in FIG. 5, the rotor supporting body11 on which the polygon mirror 19 is mounted is supported on thereceiving tool stand 31. Just before the tip end of the edges 24 of thepressure combination tool 28 contacts the boss portion 18 of the rotorsupporting body 11, the pressure combination tool 28 is subjected to avibration by the excitation source as it is pressed downward. Then, apressure joint is formed instantaneously and in the resulting structurethe pressing projection 25 is pressed out so as to overlay the settinghole. The time required for the formation of the pressure joint is just0.2 second degree.

The edge 29 pressing against the inner side of the boss portion 18 ofthe rotor supporting body 11 vibrates with a high frequency (15 KHz)having a small amplitude. Accordingly, the edge 29 can easily pierceinto the boss portion 18 of the rotor supporting body 11.

The material of the boss portion 18 of the rotor supporting body 11 isaluminum, and this aluminum material is located in response to the highfrequency vibration applied to the edge 29 and softens instantly. Thus,the edge 29 can easily pierce into the boss portion 18 of the rotorsupporting body 11 without resistance.

In an actual fitting-into or setting operation, after the edge 29 pushesforward to a predetermined depth, the edge 29 is withdrawn immediatelyand is returned to its initial position. In this regard, when the edge29 was not withdrawn, but was maintained in a stopped position where thevibration was allowed to continue, it was confirmed that the portion ofthe edge 29 in the groove 24 caused melting of the aluminum material ofthe boss portion 18, which is not desirable.

In this way, the boss portion 18 of the rotor supporting body 11 issubjected to vibration having a high frequency. The groove 24 is formedat the boss portion 18 of the rotor supporting body 11 by the piercingedge 29. By the formation of the groove 24, the outer side face of thepressing projection 25 is caused to overlay the outer periphery of theupper side edge portion of the setting hole of the polygon mirror 19.Therefore, the pressure joining method is carried out.

The upper side edge portion of the setting hole of the polygon mirror 19is chamfered to an angle of 45°, while the pressing projection 25 isinclined with an angle of 35° against a vertical axis. Therefore, in thepressure joint, as shown in FIG. 7, a connection part is formed betweenthe pressing projection 25 and the chamfering portion. In thisconnection part, the existence of a metal contact is confined to a smallrange MS.

Namely, considering a process for completing the pressure joining methodbetween the pressing projection 25 and the chamfering portion of theupper side edge of the setting hole of the polygon mirror 19 with theformation of the pressing projection 25, it is seen that the connectionbetween the pressing projection 25 and the chamfering portion progressesstarting from the range MS.

In the connection part of the range MS, the vibration having a highfrequency is mainly applied through the pressing projection 25. It hasbeen found that, due to the generated high heat, the contacting face inthe connection part is melted and produces a metal combination. Thismetal combination is equivalent to welding, and so a very strong jointis obtained. Further, to achieve the best result, it is preferable toselect an amplitude direction of the vibration along a downward pressingdirection of the pressure combination tool 28.

As stated above, the coincidence between the rotational centers of thepolygon mirror 19 and the rotor supporting body 11 can be attained bythe pressure joining method employs only pressing under appliedpressure, however the above stated coincidence between the rotationalcenters is more easily maintained by this vibrating pressure joiningmethod.

In the process where the pressing projection 25 is formed by applying avibration having a high frequency to the tool engaging the boss portion18 of the rotor supporting body 11, which is supported by the receivingtool stand from a lower face thereof, within the gap G, any leaning ormisalignment of the polygon mirror 19 is avoided. Namely, as thepressing projection 25 is pushed out toward the outer periphery thereof,the pressing projection 25 first pushes toward the upper side edgeportion of the setting hole which represents a narrow portion of the gapG. Therefore, since the narrow portion of the gap G is caused toenlarge, the centering between the polygon mirror 19 and the rotorsupporting body 11 is established. When it is minutely vibrated under avibration having a high frequency, the pressing force for correcting theleaning of the pressing projection 25 is emphasized, accordingly acentering having the extremely high accuracy can be carried out.

The pressure force applied by the edges 29 of the pressure combinationtool which is subjected to vibration is remarkably small in comparisonwith the above stated pressure joining method using only pressing. Inthe pressure joining method which uses only pressing, it is necessary toprovide a pressing force of 25 kgf per one edge. However, in thevibrating pressure combination method, it is sufficient to employ apressing force of only 2.5 kgf per one edge. In the vibrating pressurecombination method, compared with the pressure joining method which usesonly pressing, the pressure force is extremely small requiring less than1/10 the pressure, whereby a compact pressing apparatus can be obtained.

Further, in the vibrating pressure joining method, since the pressurejoining is carried out with a small pressure force, a mechanical warpaffecting the mirror face due to the pressure is small; and, regardlessof the pressure joining method under the small pressing pressure, afixing strength is attainable for the mirror sufficient to fully bearthe high speed rotation provided by the motor. The above stated factwill be explained with reference to FIG. 8.

FIG. 8 shows the results of experimentation. The vertical axis shows themirror face accuracy and the horizontal axis shows a torque valueindicating the fastening force. On the vertical axis, a regulating valuefor design is determined under a limitation of 1/4 of the lightwavelength (λ=633 nm), namely 158 nm, and this design regulation valueof the face accuracy dispersion is indicated by a D point. As amanagement limitation lower than the D point, an E point representingthe upper limit value of the face accuracy dispersion is determined; andfurther, as a management limitation lower than the E point, an F pointrepresenting a lower limit value of the face accuracy dispersion isdetermined.

On the horizontal axis, the design regulation value of the loosenesstorque dispersion is determined and indicated by an A point. Withconsideration to possible variation in the actual working, a lower limitvalue of the looseness torque dispersion is determined as a B point, andan upper limit value of the looseness torque dispersion is determined asa C point.

By this limitation, namely in an area below a horizontal line throughthe E point, above a horizontal line through the F point and bounded bythe vertical lines through the B point and the C point, a four-sidedfigure is formed. A result of a pressure joint is set to fall withinthis four-sided figure, and a condition for forming a convex shapepressure joining method is required.

As a condition for the processing, a mechanical processing tolerance ofthe component is determined and the component is processed. Bycontrolling and managing an output value and the oscillating frequencyof the supersonic excitation for forming the convex shape, a dimensionof a cut-into amount of the tip end edge portion (an edge sinking amountfrom the end face) can be made to fall within the above statedmanagement range. The characteristic of the motor installation with arotor having a mirror is confirmed and the characteristic satisfies theabove regulations.

As stated above, according to the present invention, the polygon mirrorassembly comprises the polygon mirror and a supporting stand forsupporting the polygon mirror, and the supporting stand and the polygonmirror are combined in a pressure joining manner.

With the above stated polygon mirror supporting structure, a screwmember for fastening is unnecessary, and so screw member fasteningworking and fastening adjustment also are unnecessary.

What is claimed is:
 1. A polygon mirror assembly comprising:a polygonmirror having a setting hole at a center portion thereof and regularpolygon shape plural corner portions; and a supporting stand forsupporting said polygon mirror; wherein said polygon mirror and saidsupporting stand are secured to each other by at least one pressurejoint, each pressure joint being at a position which is substantiallyaligned in a radial direction with a corner portion of said regularpolygon shape plural corner portions of said polygon mirror; saidpressure joint formed by applying a pressure force with a supersonichigh frequency to said positions to be subjected to said pressure jointof said supporting stand; wherein said pressure joint is produced byradially and symmetrically arranged plural grooves at said positions tobe subjected to said pressure joint of an upper side outer periphery ofsaid supporting stand; radially and symmetrically arranged pluralpressing projection portions are overlaid and pressed down at saidpositions to be subjected to said pressure joint against an upper sideportion of said setting hole of said polygon mirror; said pluralpressing projection portions are formed according to a formation of saidplural grooves; and material of said supporting stand is deformedoutward from an outer peripheral portion to form said plural pressingprojection portions.
 2. A polygon mirror assembly comprising:a polygonmirror having a mirror surface arranged around an outer peripherythereof, a setting hole at a center portion thereof, and regular polygonshape plural corner portions; and a supporting stand for supporting saidpolygon mirror and having a boss portion, said boss portion of saidsupporting stand being fitted into said setting hole of said polygonmirror; wherein said boss portion of said polygon mirror and saidsetting hole of said supporting stand are secured to each other by atleast one pressure joint, each said pressure joint being at said regularpolygon shape plural corner portions of said polygon mirror; saidpressure joint is formed by applying a pressure force with a supersonichigh frequency to said positions to be subjected to said pressure jointof said supporting stand; said pressure joint is produced by formingradially and symmetrically arranged plural grooves at said positions tobe subjected to said pressure joint of an upper side outer periphery ofsaid supporting stand; radially and symmetrically arranged pluralpressing projection portions are overlaid and pressed down at saidpositions to be subjected to said pressure joint against an upper sideportion of said setting hole of said polygon mirror; said pluralpressing projection portions are formed according to a formation of saidplural grooves; and material of said supporting stand is deformedoutward from an outer peripheral portion to form said plural pressingprojection portions.
 3. A polygon mirror assembly comprising:a polygonmirror having a mirror surface arranged around an outer peripherythereof and a setting hole at a center portion thereof and regularpolygon shape plural corner portions; and a supporting stand forsupporting said polygon mirror and having a boss portion, said bossportion of said supporting stand being fitted into said setting hole ofsaid polygon mirror; wherein said polygon mirror and said supportingstand are secured to each other by at least one pressure joint, witheach said pressure joint being provided at a position corresponding tosaid regular polygon shape plural corner portions of said polygonmirror; said pressure joint is formed by applying a pressure force witha supersonic high frequency to said positions to be subjected to saidpressure joint of said supporting stand to cause said plural projectionportions into pressing contact with an inner peripheral portion of saidsetting hole of said polygon mirror to form said plural projectionportions; said pressure joint is produced by forming radially andsymmetrically arranged plural grooves at said positions to be subjectedto said pressure joint of an upper side outer periphery of saidsupporting stand; said plural pressing projection portions are overlaidand pressed down at said positions to be subjected to said pressurejoint against an upper side portion of said setting hole of said polygonmirror to cause said plural projection portions into pressing contactwith an inner peripheral portion of said setting hole of said polygonmirror to form said pressure joint; said plural pressing projectionportions are formed according to a formation of said plural grooves; andmaterial of said supporting stand is deformed outward from an outerperipheral portion to form said plural pressing projection portions. 4.A polygon mirror assembly comprising:a stator; a rotor; a polygon mirrorhaving a setting hole at a center portion thereof and regular polygonshape plural corner portions; and a supporting stand for supporting saidpolygon mirror; wherein said polygon mirror and said supporting standare secured to each other by at least one pressure joint, with each saidpressure joint being provided at a position corresponding to saidregular polygon shape plural corner portions of said polygon mirror;said pressure joint is formed by applying a pressure force with asupersonic high frequency to said positions to be subjected to saidpressure joint of said supporting stand; said pressure joint is producedby forming radially and symmetrically arranged plural grooves at saidpositions to be subjected to said pressure joint of an upper side outerperiphery of said supporting stand; radially and symmetrically arrangedplural pressing projection portions are overlaid and pressed down atsaid positions to be subjected to said pressure joint against an upperside portion of said setting hole of said polygon mirror; said pluralpressing projection portions are formed according to a formation of saidplural grooves; and material of said supporting stand is deformedoutward from an outer peripheral portion to form said plural pressingprojection portions.